Pharmacokinetics and Pharmacodynamics Practice Questions

Q: A patient with normal renal function has received a daily maintenance dose of digoxin for 2 weeks. If the dosage is changed, in approximately how long would the new steady-state plasma digoxin concentration be achieved?

1 week. The time required to reach a new steady-state concentration (Css) after a change in dosage is determined by the drug's **half-life ($t_{1/2}$)** [2]. It is a fundamental pharmacokinetic principle that steady state is achieved after **4 to 5 half-lives** of a drug [2]. Digoxin has a half-life of approximately **36 to 40 hours** (roughly 1.5 days) in a patient with normal renal function [1]. Therefore, approximately **1 week** is required to reach the new steady state. Digoxin has a **narrow therapeutic index** (0.5–2 ng/mL), plasma levels should only be measured after steady state is reached (i.e., after 1 week) to ensure accuracy [1].

Q: Exogenous adrenaline is metabolized by:

COMT. **Explanation:** The metabolism of catecholamines (Adrenaline, Noradrenaline, and Dopamine) involves two primary enzymes: **Catechol-O-methyltransferase (COMT)** and **Monoamine oxidase (MAO)**. **Why COMT is correct:** COMT is responsible for the O-methylation of catecholamines. When adrenaline is administered **exogenously**, it is primarily metabolized by COMT in the liver and kidneys. COMT acts on the catechol ring, converting adrenaline into metanephrine. In contrast, MAO is primarily located in the mitochondria of neuronal terminals and is more significant for the reuptake and degradation of endogenous (neuronal) catecholamines. **Why other options are incorrect:** * **AChE (Acetylcholinesterase):** This enzyme is specific for the rapid hydrolysis of **Acetylcholine** at cholinergic synapses. It has no role in catecholamine metabolism. * **Decarboxylase (L-amino acid decarboxylase):** This enzyme is involved in the **synthesis** (not metabolism) of catecholamines, specifically converting L-Dopa to Dopamine. * **Acetyl transferase:** This enzyme is involved in the metabolism of drugs like Isoniazid, Hydralazine, and Procainamide (via acetylation), but not adrenaline. **High-Yield Clinical Pearls for NEET-PG:** * **VMA (Vanillylmandelic acid):** The final common metabolic product of both adrenaline and noradrenaline. Elevated urinary VMA levels are a diagnostic marker for **Pheochromocytoma**. * **Metanephrines:** These are the intermediate metabolites (via COMT). Measuring plasma free metanephrines is currently considered the most sensitive screening test for Pheochromocytoma. * **COMT Inhibitors:** Drugs like **Entacapone** and **Tolcapone** are used in Parkinson’s disease to prevent the peripheral breakdown of Levodopa.

Q: In the given pharmacodynamic curve, point A corresponds to:

ED50. ***ED50*** - Point A on the **dose-response curve** represents the dose that produces **50% of the maximum response**, which is the definition of **ED50**. - It is located at the **intersection** of the 50% response level with the curve, read directly from the **x-axis (dose axis)**. *Potency* - **Potency** refers to the overall **position of the curve** on the x-axis, not a specific point like A. - A more potent drug has its entire curve **shifted to the left**, requiring lower doses for the same effects. *Efficacy* - **Efficacy** represents the **maximum response** achievable by a drug, shown as the **plateau height** of the curve. - Point A is on the **dose axis**, not at the maximum response level, so it cannot represent efficacy. *LD50* - **LD50** is the **lethal dose** for 50% of the population, derived from **quantal dose-response curves**. - This parameter is used for **toxicity studies**, not therapeutic effect measurements shown in graded dose-response curves.

Q: Receptor-mediated action is not seen in which of the following?

Antacids. **Explanation:** The core concept tested here is the **mechanism of drug action**. Most drugs act by binding to specific **receptors** (proteins), but some exert their effects through non-receptor-mediated physical or chemical processes. **Why Antacids are the correct answer:** Antacids (e.g., Aluminum hydroxide, Magnesium hydroxide) act through a **purely chemical neutralization reaction**. They are weak bases that react directly with gastric hydrochloric acid (HCl) to form salt and water, thereby increasing the pH of the gastric contents. This process does not involve binding to any cellular receptor. **Analysis of Incorrect Options:** * **Antipsychotics:** These drugs act primarily by blocking **Dopamine (D2) receptors** in the mesolimbic pathway of the brain. * **Benzodiazepines:** These act as positive allosteric modulators of the **GABA-A receptor** complex, increasing the frequency of chloride channel opening. * **Alcohol (Ethanol):** While alcohol has complex effects, its primary sedative-hypnotic actions are mediated through the enhancement of **GABA-A receptors** and inhibition of NMDA glutamate receptors. **High-Yield Clinical Pearls for NEET-PG:** Other examples of **Non-Receptor Mediated Mechanisms** frequently asked in exams include: 1. **Physical Action:** Adsorbents (Activated Charcoal), Osmotic laxatives (Lactulose, Magnesium sulfate), and Osmotic diuretics (Mannitol). 2. **Chemical Action:** Chelating agents (EDTA, Dimercaprol) which bind to heavy metals. 3. **Counterfeit Incorporation:** Purine/Pyrimidine analogues (Anticancer drugs) that get incorporated into DNA. 4. **Enzyme Inhibition:** Drugs like Neostigmine (AChE) or Captopril (ACE).

Q: Volume of distribution of a drug is given by?

Vd = Dose administered i.v. / Plasma concentration. **Explanation:** **1. Why Option A is Correct:** The **Apparent Volume of Distribution (Vd)** is a theoretical volume that relates the total amount of drug in the body to the concentration of the drug measured in the plasma. It is calculated using the formula: **Vd = Total amount of drug in the body (Dose) / Plasma concentration (C)** When a drug is administered intravenously (i.v.), the entire dose enters the systemic circulation immediately. Therefore, Vd represents the space required to contain the drug if it were distributed homogeneously at the same concentration found in the plasma. **2. Why Other Options are Incorrect:** * **Option B:** This describes a ratio related to the Therapeutic Index or safety margin, not distribution. * **Option C:** While lipid solubility *influences* Vd (high lipid solubility usually leads to a high Vd), it is not a mathematical component of the formula. * **Option D:** This is a distractor. While Vd is used to calculate half-life ($t_{1/2} = 0.693 \times Vd / CL$), the formula provided is mathematically incorrect. **3. NEET-PG High-Yield Clinical Pearls:** * **Loading Dose:** Vd is the primary determinant of the loading dose ($LD = Vd \times Target\ Plasma\ Concentration$). * **Low Vd ( 40L):** Indicates the drug is sequestered in peripheral tissues/fat (e.g., **Chloroquine, Digoxin**). * **Dialysis:** Drugs with a high Vd cannot be efficiently removed by hemodialysis because most of the drug is outside the plasma. * **Plasma Protein Binding:** Drugs with high plasma protein binding tend to have lower Vd.

Q: Which of the following drugs has the maximum chance of absorption from the gastric mucosa?

Diclofenac sodium. ### Explanation The absorption of a drug across biological membranes is primarily governed by its **degree of ionization**, which depends on the drug's pKa and the pH of the surrounding medium. According to the **pH Partition Hypothesis**, only the non-ionized (lipid-soluble) form of a drug can easily cross lipid membranes. **1. Why Diclofenac Sodium is Correct:** Diclofenac is a **weakly acidic drug** (pKa ≈ 4.0). In the highly acidic environment of the stomach (pH 1–2), weak acids remain largely in their **non-ionized, protonated form**. This makes them lipid-soluble and allows for significant absorption directly through the gastric mucosa. **2. Why the Other Options are Incorrect:** * **Morphine sulfate, Hyoscine hydrobromide, and Quinine dihydrochloride** are all **weakly basic drugs**. * In the acidic gastric environment, basic drugs become **highly ionized** (protonated). * Ionized molecules are water-soluble but lipid-insoluble; therefore, they cannot cross the gastric mucosal barrier effectively. These drugs are primarily absorbed later in the alkaline environment of the small intestine. **3. High-Yield Clinical Pearls for NEET-PG:** * **General Rule:** Acidic drugs (e.g., Aspirin, Barbiturates, NSAIDs like Diclofenac) are absorbed better in acidic media (stomach). Basic drugs (e.g., Atropine, Morphine, Amphetamines) are absorbed better in alkaline media (intestine). * **Surface Area Factor:** Despite the chemistry, the **small intestine** remains the major site of absorption for *most* drugs (including weak acids) due to its massive surface area compared to the stomach. * **Ion Trapping:** This principle is used clinically in toxicology. For example, alkalinizing the urine with sodium bicarbonate "traps" acidic drugs like Aspirin in the renal tubules, enhancing their excretion.

Q: Which of the following is FALSE regarding non-competitive antagonism?

The drug needs to be structurally similar to the substrate.. ### Explanation In pharmacology, understanding the distinction between competitive and non-competitive antagonism is high-yield for NEET-PG. **Why Option B is the Correct (False) Statement:** In **non-competitive inhibition**, the antagonist does not compete for the same binding site as the substrate. Instead, it binds to an **allosteric site** (a different location on the enzyme or receptor). Because it does not target the active site, the antagonist **does not need to be structurally similar** to the substrate. Structural similarity is a hallmark of *competitive* inhibition, where the drug "mimics" the substrate to fit into the active site. **Analysis of Other Options:** * **Option A (True):** Non-competitive inhibitors bind to allosteric sites, inducing a conformational change in the enzyme that reduces its catalytic activity. * **Option C (True):** Since the inhibitor and substrate are not competing for the same spot, adding more substrate cannot "wash out" or displace the inhibitor. Therefore, the inhibition is **insurmountable**, and the **Vmax is decreased**. * **Option D (True):** In non-competitive inhibition, the affinity of the remaining functional enzymes for the substrate typically remains unchanged. Therefore, the **Km (Michaelis constant) remains the same**. **High-Yield Clinical Pearls for NEET-PG:** * **Competitive Inhibition:** Km increases, Vmax remains unchanged (e.g., Statins vs. HMG-CoA). * **Non-Competitive Inhibition:** Km remains unchanged, Vmax decreases (e.g., Digoxin acting on Na+/K+ ATPase). * **Irreversible Inhibition:** Often grouped with non-competitive kinetics because Vmax decreases (e.g., Aspirin inhibiting COX, Organophosphates inhibiting AChE). * **Key Graph:** On a Lineweaver-Burk plot, non-competitive inhibitors show lines that intersect on the negative X-axis (same -1/Km).

Q: Dosage of a drug is determined by all of the following factors except?

Half-life. To determine the correct dosage of a drug, we must distinguish between the **Loading Dose** and the **Maintenance Dose**. **Why Half-life is the correct answer:** The **half-life ($t_{1/2}$)** of a drug determines the **dosing interval** (how often the drug is given) and the time required to reach **steady-state concentration**, but it does not directly determine the *amount* (dosage) of the drug itself. Dosage is primarily calculated based on Clearance and Volume of Distribution. **Explanation of Incorrect Options:** * **Volume of Distribution ($V_d$):** This is the primary determinant of the **Loading Dose**. Drugs with a high $V_d$ sequester into tissues, requiring a larger initial dose to achieve the desired target plasma concentration ($LD = V_d \times C_p$). * **Excretion/Clearance:** The rate of drug elimination (Clearance) is the primary determinant of the **Maintenance Dose**. If excretion is impaired (e.g., renal failure), the dose must be reduced to avoid toxicity ($MD = CL \times C_{ss}$). * **Lipid Solubility:** This physicochemical property dictates the $V_d$. Highly lipid-soluble drugs cross biological membranes easily and distribute widely into adipose tissue, necessitating dosage adjustments based on body composition. **NEET-PG High-Yield Pearls:** 1. **Loading Dose** depends on **$V_d$** (Targeting the "tank" size). 2. **Maintenance Dose** depends on **Clearance** (Targeting the "leak" rate). 3. **Steady State** is reached after **4–5 half-lives**, regardless of the dose or frequency, provided the infusion/dosing is constant. 4. **Bioavailability (F):** Must be considered for oral dosing ($LD = \frac{V_d \times C_p}{F}$).

Question 1

A patient with normal renal function has received a daily maintenance dose of digoxin for 2 weeks. If the dosage is changed, in approximately how long would the new steady-state plasma digoxin concentration be achieved?

Accepted Answer

1 week

Answer

2 days

Answer

2 weeks

Answer

4 weeks

Question 2

Exogenous adrenaline is metabolized by:

Accepted Answer

COMT

Answer

AChE

Answer

Decarboxylase

Answer

Acetyl transferase

Question 3

In the given pharmacodynamic curve, point A corresponds to:

Accepted Answer

ED50

Answer

Potency

Answer

Efficacy

Answer

LD50

Question 4

Receptor-mediated action is not seen in which of the following?

Accepted Answer

Antacids

Answer

Alcohol

Answer

Antipsychotics

Answer

Benzodiazepines

Question 5

What is the plasma half-life of aspirin?

Accepted Answer

Longer for anti-inflammatory doses compared to analgesic doses

Answer

Independent of dose

Answer

Shorter for anti-inflammatory doses compared to analgesic doses

Answer

Can be increased by alkalinizing the urine

Question 6

A patient is prescribed propranolol with the following pharmacokinetic parameters: Volume of Distribution (Vd) = 300 L/70 kg, Clearance (C1) = 700 mL/min, and oral bioavailability (f) = 0.25. What is the oral dose required to achieve a plasma concentration of 20 mg/L?

Accepted Answer

24 mg

Answer

4 mg

Answer

12 mg

Answer

48 mg

Question 7

Volume of distribution of a drug is given by?

Accepted Answer

Vd = Dose administered i.v. / Plasma concentration

Answer

Vd = Maximum tolerated dose / Dose administered i.v.

Answer

Vd = Dose administered i.v. / Total lipid solubility

Answer

Vd = t 1/2 / Dose administered i.v.

Question 8

Which of the following drugs has the maximum chance of absorption from the gastric mucosa?

Accepted Answer

Diclofenac sodium

Answer

Morphine sulfate

Answer

Hyoscine hydrobromide

Answer

Quinine dihydrochloride

Question 9

Which of the following is FALSE regarding non-competitive antagonism?

Accepted Answer

The drug needs to be structurally similar to the substrate.

Answer

The drug binds to an allosteric site of the enzyme.

Answer

The inhibition cannot be overcome by increasing the substrate concentration.

Answer

The Km remains the same.

Question 10

Dosage of a drug is determined by all of the following factors except?

Accepted Answer

Half-life

Answer

Volume of distribution

Answer

Lipid solubility

Answer

Excretion of the drug

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

On this page

Practice by Chapter

Want unlimited practice?